Sample throughput in a counting system is most affected by efficiency and background. Together these determine the sensitivity of the system. Low-background systems are often specified using system performance data and sample performance data. It is important to understand both to compare systems. System performance is the best possible performance available for the low-background system. System performance is measured by minimizing sample effects as much as possible. In most cases, system performance cannot be changed, so selecting a model with the best system performance is critical to sample throughput.

Sample background and efficiency depend on preparation techniques, geometry, and content. All are variable and are controlled to some extent by the user. For example, a sample can be placed close to the detector to improve efficiency, or further away from the detector to reduce the chance of detector window breakage. When you compare systems, it is important to compare system performance to system performance, and compare sample performance to sample performance. This is relatively easy if you calculate a “Figure of Merit” using equivalent performance specifications for each system. Figure of Merit is calculated by the following equation:

FOM = Figure of Merit
E = Efficiency in percent for the sample type of interest
B = Background in counts per minute for the system

Given the same sample and counting sensitivity requirements, a system with twice the Figure of Merit will have twice the sample throughput. These dramatic increases in sensitivity translate directly into shorter count times in the laboratory.

The best possible sensitivity is achieved using a windowless system with a low system background. Using the efficiency figures for 90Sr below, a typical beta background of 0.7 cpm, and 5/16 inch sample depth, a PIC IPC-650 system has a Figure of Merit of:

In practical terms, this means that the windowless system will count the same sample to the same sensitivity (MDA), in a fraction of the time of the system with a window installed.

This efficiency data was collected on a Protean Instrument IPC-650 windowed/windowless system. The sources are all calibrated and the active area is 47 mm. Each source was counted at four depths, “top,” 1/8, 1/4, and 5/16 inches. Top placement is done by using a 1/8 inch planchet inverted in a 1/8 inch insert. The source is then at the minimum distance possible (approximately 1/16 inch). The right table is for a system with a standard 80 micrograms/cc2 window installed. The left table is for the same system with the window removed. A system with a window shows a great change in efficiency, depending on sample depth. This effect is the same regardless of system type. A windowless system shows a substantial increase in efficiency compared to a windowed system, and very little efficiency change due to sample depth. The efficiencies listed for the system with window installed are the same for all Protean alpha/beta counting systems using a gas flow proportional detector with a detector window.

Window versus Windowless Detector Efficiency at Various Sample Depths

Note: All sources used are NIST traceable and 47 mm in active diameter.